1. Field of the Invention
The present invention relates to a crystalline titanium dioxide powder having a variable sintering stability, a process for preparing the crystalline titanium dioxide powder, and a catalyst and/or a catalyst support comprising the crystalline titanium dioxide powder.
2. Discussion of the Background
Titanium dioxide may be produced by pyrogenic processes including flame oxidation and/or flame hydrolysis. In flame oxidation, a titanium dioxide precursor (e.g., titanium tetrachloride) is oxidized with oxygen according to the following equation (1). In flame hydrolysis, titanium dioxide is formed by hydrolysis of the titanium dioxide precursor, with the requisite water for hydrolysis obtained from combustion of a fuel gas (e.g., hydrogen) with oxygen according to the following equation (2).TiCl4+O2→TiO2+2Cl2  (1)TiCl4+2H2O→TiO2+4HCl  (2)
Titanium dioxide is generally present in the crystalline forms of anatase and rutile. In the references described below, the primary focus is on maximizing the proportion of anatase.
WO 96/06803 describes a process for preparing titanium dioxide by a combination of flame oxidation and flame hydrolysis. Gaseous titanium tetrachloride and oxygen are mixed in a reaction zone and the mixture is heated in a flame produced by combustion of a hydrocarbon as a fuel gas. Titanium dioxide powder having a BET surface area of about 70-110 m2/g and a rutile content of less than 10% is described.
Powder Technology, Vol. 86, No. 3, pp. 87-93 (March 1996) describes a flame oxidation process carried out in a diffusion reactor. No rutile is detected in a BET surface area of about 80-120 m2/g.
DE-A-102004055165 describes a titanium dioxide powder that is prepared by flame hydrolysis and has a BET surface area of 20-200 m2/g. Although the anatase/rutile ratio is said to be able to be varied in a range from 2:98 to 98:2 at a given BET surface area, the examples show that the proportion of anatase clearly dominates. In a BET surface area of about 70-100 m2/g, the anatase content is at least 90%.
JP-A-10251021 describes three titanium dioxide powders obtained by flame oxidation, wherein the three titanium dioxide powders have a BET surface area of 3.2 m2/g, 28 m2/g and 101 m2/g and a rutile content of 97%, 32% and 15%, respectively.
As evidenced by the aforementioned references, there is a pronounced interest in pyrogenically produced titanium dioxide and the complex processes associated therewith.